libxenlight: tests a lots more of xl return value inside the library

libxenlight: tests a lots more of xl return value inside the library
and in xl.

introducing a domain where the xenguest build function has fail, lead
to having xenstored receiving SIGBUS, since it's trying to access some
of the domain's memory, which haven't been properly allocated. (it
doesn't seems to be a way to make xenstored more robust to this though
since xc_map_foreign_range just succeed).

make xl a lot more robust regarding all those random errors possible.

Signed-off-by: Vincent Hanquez <vincent.hanquez@eu.citrix.com>